1. Field of the Invention
The invention relates to a magnetized nut for fastening a compressor wheel of an exhaust turbocharger to the turbo shaft, the nut having a body made from a non-magnetic material, which forms a hollow space for accommodating a magnetic material, and to a method for manufacturing the magnetized nut.
2. Related Art
The power output generated by an internal combustion engine depends upon the air mass and the quantity of fuel that can be delivered to the engine for combustion. To boost the power output of the internal combustion engine it is necessary to increase the quantity of delivered combustion air and fuel. This increase in the power output is achieved, in the case of a naturally aspirated engine, by an increase in swept volume or by increasing the engine speed. An increase in swept volume leads to heavier, dimensionally larger and hence more expensive internal combustion engines. The increase in engine speed, particularly in larger internal combustion engines, brings considerable problems and disadvantages and is limited for technical reasons.
A solution commonly used to boost the power output of an internal combustion engine is supercharging. By this we mean the pre-compression of the combustion air by an exhaust turbocharger or also by a compressor mechanically driven by the engine. An exhaust turbocharger basically comprises a flow compressor and a turbine, which are connected to a common turbo shaft and rotate at the same speed. The turbine converts the normally wasted energy of the exhaust gas into rotational energy that drives the compressor.
The compressor draws fresh air in and delivers the compressed air to the individual cylinders of the engine. A greater quantity of fuel can be fed to the larger quantity of air in the cylinders, so that the internal combustion engine delivers more power. This furthermore has a beneficial effect on the combustion process, so that the internal combustion engine achieves a better overall efficiency. In addition it is possible to design an internal combustion engine supercharged by a turbocharger with an extremely favorable torque profile. Standard production engines of vehicle manufacturers can be substantially improved through the use of an exhaust turbocharger without making major design changes to the internal combustion engine. Supercharged internal combustion engines generally have a lower specific fuel consumption and lower pollutant emissions. Turbocharged engines are furthermore quieter than naturally aspirated engines of equal output, since the exhaust turbocharger itself acts as an additional silencer.
For internal combustion engines having a larger operating speed range, such as automobile internal combustion engines, a higher boost pressure is required even at low engine speeds. For this purpose a boost pressure control valve, a so-called wastegate valve, is introduced. By selecting a corresponding turbo housing, a high boost pressure is rapidly built up even at low engine speeds. As the engine speed increases the boost pressure control valve (wastegate valve) then limits the boost pressure to a constant value. Alternatively turbochargers having a variable turbine geometry (VTG) are used. In these turbochargers the boost pressure is regulated by varying the turbine geometry.
With an increasing quantity of exhaust gas the maximum admissible combined speed of the turbine wheel, the compressor wheel, and turbo shaft, also referred to as the rotor of the turbocharger, may be exceeded. If the maximum admissible speed of the rotor is exceeded, the rotor would be destroyed, which is tantamount to a total loss of the turbocharger. Small, modern turbochargers with significantly smaller turbine and compressor wheel diameters, which have an improved rotational acceleration response due to their considerably smaller mass moment of inertia, are particularly susceptible to the problem associated with an exceeding of the admissible top speed. Depending on the type of turbocharger, simply exceeding the rotational speed limit by about 5% leads to complete destruction of the turbocharger.
Boost pressure control valves, which in the state of the art are actuated by a signal resulting from the boost pressure generated, have proved successful in limiting the speed. If the boost pressure exceeds a predefined threshold, the boost pressure control valve opens and causes a portion of the exhaust gas mass flow to bypass the turbine. Due to the reduced exhaust gas mass flow this takes up less power and the compressor output diminishes a similar degree. The boost pressure and the speed of the turbine wheel and the compressor wheel are reduced. This control reaction is relatively slow because the pressure build-up when the rotor exceeds the admissible speed ensues with a time lag. For this reason, particularly in the highly dynamic range (load reversal), the speed control for the turbocharger with boost pressure monitoring must intervene through a correspondingly premature reduction in boost pressure, which leads to an efficiency loss.
DE 10 2006 044 667.4 discloses a turbocharger having an element generating a magnetic field, in which the speed of the turbo shaft is registered by an active sensor element. The element generating the magnetic field is embodied as a magnetized nut for fastening a compressor wheel of an exhaust turbocharger to the turbo shaft. The magnetized nut has a body of a non-magnetic material, which forms a hollow space for accommodating a magnetic material. The magnetic material takes the form of a sintered permanent magnet, which is adhesively bonded or welded into the hollow space. Bonding or welding the permanent magnet in is intricate and leads to unbalanced mass distributions in the magnetized nut. Since the nut rotates together with the turbo shaft at very high speed, even minute defects in the equal distribution of the mass have an extremely detrimental effect.